Position sensor with improved magnetic circuit

ABSTRACT

A rotary position sensor connected to the butterfly valve of an internal combustion engine, the sensor having a dual magnet structure interconnected by a pole piece having a generally C-shaped cross section forming a varying dimension air gap. The magnets, the pole piece and the air gap define a closed magnetic circuit. A Hall effect sensor is fixedly mounted in the air gap and is exposed to a well defined but varying magnetic field. Through the use of specific magnetic materials and sloping, curved surfaces, a precise yet tolerance friendly magnetic circuit is produced so that the sensor produces signals as a function of the angular positions of the magnets.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to rotary or angular position sensors which areboth durable and precise for application in rugged and demandingenvironments, particularly for application with internal combustionengines.

2. Description of the Prior Art

There are a variety of known techniques for angular position sensing.Optical, electrical, electrostatic and magnetic fields are all used withapparatus to measure position.

There are many known apparatus for using these energies for sensing. Afew of the known apparatus are resistive contacting sensors, inductivelycoupled ratio detectors, variable reluctance devices, capacitivelycoupled ratio detectors, optical detectors using the Faraday effect,photo-activated ratio detectors, radio wave directional comparators, andelectrostatic ratio detectors. There are many other known detectors, toonumerous to mention herein.

These detection methods tend to each offer much value for one or moreapplications, but none meet all application requirements for allposition sensing applications. The limitations may be due to cost,sensitivity to particular energies and fields, resistance tocontamination and environment, stability, ruggedness, linearity,precision, or other similar factors.

Transportation applications generally, and specifically automotiveapplications, are very demanding. Temperatures may rise to 150 degreesCentigrade or more, with road contaminants such as salt and dirtsplashing upon the engine compartment. This may occur while the engineis still extremely hot from operation. At the other extreme, an engineis expected to perform in the most northern climates without fault, andwithout special preheating.

Present throttle position sensors are manufactured using a resistivesensor combined with a sliding contactor structure. The sliding contactserves to "tap" the resistor element and provide a voltage proportionalto position. The resistive sensor has proven to offer the greatestperformance for cost in throttle position sensing applications,unmatched by any other technology to date. However, the resistivethrottle position sensors are not without limitation.

An automotive position sensor must endure many millions or even billionsof small motions referred to in the industry as dithers. These dithersare the result of mechanical motion and vibration carried into theposition sensor. Additionally, during the life of a throttle positionsensor, there may be a million or more full stroke cycles of motion. Inresistive sensors, these motions can affect signal quality.

In spite of this shortcoming, throttle position sensors are resistivesensors. Over the years, efforts at improving the contactor-elementinterface have vastly improved the performance of these devices. Similarimprovements in packaging and production have maintained cost advantage.A replacement component must be able to meet throttle position sensorperformance requirements while offering similar price advantage.

The combination of temperature extremes and contamination to which anautomotive sensor is exposed causes the industry to explore very ruggedand durable components. One particular group of sensors, those whichutilize magnetic energy, are rapidly being accepted into these demandingapplications. This is because of the inherent insensitivity of themagnetic system to contamination, together with durabilitycharacteristic of the components.

Applying magnetic sensing to tone wheels for applications such asanti-lock braking and ignition timing has been a relatively easy task.The impulse provided by the tone wheel is readily detected through allconditions, with very simple electronic circuitry.

Magnetic throttle position sensors, particularly those using Hall effectIC detectors, are also aggressively being pursued. The industry believesthese sensors will offer advantages over the present resistivetechnology. However, prior to the present invention, none of thesesensors were able to offer the necessary combination of low cost,reliability, and precision output.

Magnetic circuits offer admirable performance upon exposure to the usualmoisture and dirt contaminants. However, linearity and tight tolerancesare another issue. Sensors are subjected to both radial and axial forcesthat change the alignment of the rotor portion of the sensor withrespect to the stationary portion (stator). Somewhere in the system isat least one bearing, and this bearing will have a finite amount ofplay, or motion. That play results in the rotor moving relative to thestator.

Unfortunately, magnetic circuits of the prior art tend to be verysensitive to mechanical motion between the rotor and stator. As noted,this motion may be in an axial direction parallel to the axis ofrotation, or may be in a radial direction perpendicular to the axis, ora combination thereof.

Typical magnetic circuits use one or a combination of magnets togenerate a field across an air gap. The magnetic field sensor, be this aHall effect device or a magnetoresistive material or some other magneticfield sensor, is then inserted into the gap. The sensor is alignedcentrally within the cross-section of the gap. Magnetic field lines arenot constrained anywhere within the gap, but tend to be most dense andof consistent strength centrally within the gap. Various means may beprovided to vary the strength of the field monitored by the sensor,ranging from shunting the magnetic field around the gap to changing thedimensions of the gap.

Regardless of the arrangement and method for changing the field aboutthe sensor, the magnetic circuit faces several obstacles which haveheretofore not been overcome. Movement of the sensor relative to thegap, which is the result of axial and radial play between the rotor andstator, will lead to a variation in field strength measured by thesensor. This effect is particularly pronounced in Hall effect,magneto-resistive and other similar sensors, where the sensor issensitive about a single axis and insensitive to perpendicular magneticfields.

The familiar bulging of field lines jumping a gap illustrates this,where a Hall effect sensor not accurately positioned in the gap willmeasure the vector fraction of the field strength directly parallel tothe gap. In the center of the gap, this will be equal to the full fieldstrength. The vector fraction perpendicular thereto will be ignored bythe sensor, even though the sum of the vectors is the actual fieldstrength at that point. As the sensor is moved from the center of thegap, the field begins to diverge, or bulge, resulting in a greaterfraction of the field vector being perpendicular to the gap. Since thiswill not be detected by the sensor, the sensor will provide a reading ofinsufficient magnitude.

In addition to the limitations with regard to position and fieldstrength, another set of issues must be addressed. A position sensor ofvalue in the transportation industry must be precise in spite offluctuating temperatures. In order to gain useful output, a magnet mustinitially be completely saturated. Failure to do so will result inunpredictable magnet performance. However, operating at completesaturation leads to another problem referred to in the trade asirreversible loss. Temperature cycling, particularly to elevatedtemperatures, permanently decreases the magnetic output.

A magnet also undergoes aging processes not unlike those of othermaterials, including oxidation and other forms of corrosion. This iscommonly referred to as structural loss. Structural and irreversibleloss must be understood and dealt with in order to provide a reliabledevice with precision output.

Another significant challenge in the design of magnetic circuits is thesensitivity of the circuit to surrounding ferromagnetic objects. Fortransportation applications a large amount of iron or steel may beplaced in very close proximity to the sensor. The sensor must notrespond to this external influence.

The prior art is illustrated, for example, by Tomczak et al in U.S. Pat.No. 4,570,118. Therein, a number of different embodiments areillustrated for forming the magnetic circuit of a Hall effect throttleposition sensor. The Tomczak et al disclosure teaches the use of asintered samarium cobalt magnet material which is either flat, arcuate,and slightly off-axis, or in second and third embodiments, rectangularwith shaped pole pieces. The last embodiment is most similar to thepresent invention, where there are two shaped magnets of oppositepolarity across an air gap of varying length.

No discussion is provided by Tomczak et al for how each magnet ismagnetically coupled to the other, though from the disclosure it appearsto be through the use of an air gap formed by a plastic molded carrier.Furthermore, no discussion is provided as to how this magnetic materialis shaped and how the irreversible and structural losses will bemanaged. Sintered samarium cobalt is difficult to shape with any degreeof precision, and the material is typically ground after sintering. Thegrinding process is difficult, expensive and imprecise. The device maybe designed to be linear and precise at a given temperature and a givenlevel of magnetic saturation, presumably fully saturated. However, sucha device would not be capable of performing in a linear and precisemanner, nor be reliable, through the production processes, temperaturecycling and vibration realized in the transportation environment.

Furthermore, devices made with this Tomczak et al design are highlysusceptible to adjacent ferromagnetic objects. The variation in adjacentferromagnetic material from one engine to the next will serve to distortthe field and adversely affect both linearity and precision. The openmagnetic circuit not only adversely affects sensitivity to foreignobjects, but also sensitivity to radiated energies, commonly referred toas Electro-Magnetic Interference (EMI or EMC).

The Tomczak et al embodiments are very sensitive to bearing play. Thecombination of an open magnetic circuit and radially narrow permanentmagnet structure provides no tolerance for motion in the bearing system.This motion will be translated into a changing magnetic field, since thearea within the gap in which the field is parallel and of consistentmagnetic induction is very small. Ratajski et al in U.S. Pat. No.3,112,464 illustrate several embodiments of a brushless Hall effectpotentiometer. In the first embodiment they disclose a shaped, radiallymagnetized structure which varies an air gap between the magneticstructure and a casing, not unlike the last embodiment of the Tomczak etal patent mentioned above. However, there is no provision for radial oraxial motion of the magnet carried upon the rotor. Furthermore, thelarge magnetic structure is difficult to manufacture and relativelyexpensive.

Wu in U.S. Pat. No. 5,159,268 illustrates a shaped magnet structuresimilar to Ratajski et al. The structure illustrated therein suffersfrom the same limitations as the Ratajski et al disclosure.Additionally, the device of the Wu disclosure offers no protection fromextraneous ferromagnetic objects.

Alfors in U.S. Pat. No. 5,164,668 illustrates a sensor less sensitive toradial and axial play. The disclosed device requires a large shapedmagnet for precision and linearity. The size of the magnet structureplaces additional demand upon the bearing system. No discussion thereinaddresses magnet materials, methods for compensating for irreversibleand structural losses, or shielding from extraneous ferromagneticobjects. The combination of large magnet, enhanced bearing structure,and added shielding combine to make a more expensive package.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned limitations of theprior art and perceived barriers to the use of a linear type Hall effectthrottle position sensor through the use of a special geometry magneticstructure. The present invention is a combination including a rotarysensor for use with an internal combustion engine comprising a throttleconnected to the engine, a rotatable magnetic circuit assembly connectedto the throttle so that rotation of the throttle causes rotation of themagnetic circuit assembly, the assembly including a first magnet, asecond magnet, a magnetically permeable pole piece connecting themagnets, and a varying dimension air gap defined between the magnets,the magnets being structured and dimensioned to provide the varyingdimension air gap and to form a variable magnetic field therebetween,the assembly being connected to the engine and rotatable about an axis,the axis being generally parallel to the magnetic field coupled betweenthe magnets, and a magnetic field sensing means positioned in the airgap between the magnets for sensing the variable magnetic field in theair gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a portion of the preferred embodiment of theinvention from a top view with the cover removed for clarity.

FIG. 2 illustrates the preferred embodiment of FIG. 1 from across-sectional view taken along line 2' of FIG. 1.

FIG. 3 illustrates a schematic view of the magnet and Hall effect devicestructure.

FIG. 4 illustrates an alternative magnetic structure from a perspectiveview.

FIG. 5 illustrates the embodiment of FIG. 4 from an elevational view.

FIG. 6 illustrates the embodiment of FIG. 4 from a top plan view.

FIG. 7 illustrates a top view of an alternative embodiment cover.

FIG. 8 illustrates an elevational cross-section view taken along line8--8 of FIG. 7.

FIG. 9 is a diagrammatic view of an iternal combustion engine, athrottle body/butterfly and their connection to the inventive sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrated by top view with cover removed in FIG. 1 and bycross-section in FIG. 2 is the preferred embodiment in accord with thepresent invention. Therein, a rotary sensor is designated generally bythe numeral 100. The sensor includes a housing 300 and a magneticstructure of assembly 200 of arcuate periphery and generally "c"-shapedcross section mounted to the housing. Magnet structure 200 includestherein a magnetically permeable pole piece 210, shaped magnets 212 and214, an air gap 216 and is supported by a base on molded rotor cup 220.

Pole piece 210 is bonded to magnets 212 and 214 such that the air gap isformed between and is bordered by the magnets. This use of two magnetssubstantially reduces loss through the air gap which otherwise occurswith only a single magnet. A closed magnetic circuit which is completedby pole piece 210 improves performance by being less sensitive tobearing play and less sensitive to external ferromagnetic objects. Aclosed magnetic circuit exists, for the purposes of this disclosure,when the external flux path of a permanent magnet is confined with highpermeability material. Air is understood to be low permeabilitymaterial. The pole piece 210, the magnets 212 and 214 and the air gap216 form the closed magnetic circuit. Pole piece 210 further reduces thesize of magnets 212 and 214 required, and may be manufactured frommolded or sintered metals. More preferably, pole piece 210 is formedfrom sheet steels such as ANSI 430 stainless steel. As shown in FIGS. 1and 2, the pole piece has a generally pie shape configuration in planview and has a radium from an axis 250 to outer edges 261, 263.

Shaped magnets 212 and 214 are preferably formed by molding magneticmaterials 10 such as bonded ferrite. Bonded ferrite offers both asubstantial cost advantage and also a significant advantage over othersimilar magnetic materials in structural loss due to corrosion and otherenvironmental degradation. Other magnetic materials may be suitable, aswill be determined by one skilled in the art.

Magnets 212 and 214 should extend substantially from the outer edges261, 263 of pole piece 210 to a region very close to, or, designallowing, in line with the axis of rotation 250. This large extension ofmagnets 212 and 214 in the radial direction greatly reduces the effectsof radial motion of magnetic structure 200.

Additionally, magnets 212 and 214 are formed with lip structures 474 and472 as illustrated best in FIG. 2. These formations extend out beyondand partially around pole piece 210. The lips 472 and 474 serve toexpand the "sweet zone" of operation of Hall effect device 510, byforcing a larger area of linear magnetic field lines passing through theair gap and coupled between magnets 212 and 214. This larger area oflinear field lines directly corresponds to greater tolerance for bothradial and axial play.

Molded rotor cup 220 includes a surface 223 which is coupled to athrottle body/butterfly valve 102 of an internal combustion engine 104by a shaft 106 as diagrammatically shown in FIG. 9. Molded rotor cup 220then rotates about the axis identified as 250 in FIG. 1 and carriestherewith the remainder of magnet structure 200. Molded rotor cup 220 isretained by housing 300, seal 350, helical spring 360 and cover 310.

Cover 310 engages with housing 300 and may, for example, beultrasonically welded in place. Cover 310 is strengthened againstwarpage and deflection through the formation of ribs 312.

Within the gap formed by magnets 212 and 214 is a hybrid circuitsubstrate 500 carrying thereon the Hall effect device 510. Hall effectdevice 510 should be positioned somewhere between the outer edges 265,267 of magnets 212 and 214 and the inner ends 511, 513 of the magnetsnear axis 250, but not particularly close to either the edges or theends, so as to avoid the field bulging effect mentioned earlier.

Hybrid substrate 500 may be attached by heat staking or other similarmethod to the housing 300. Hybrid substrate 500 additionally carriesthereon electrical circuitry within tray 520. This tray 520 acts as acontainer into which appropriate potting compounds may be placed toprovide all necessary environmental protection to the associatedcircuitry. Tray 520 should be electrically grounded for protectionagainst radiated fields (EMI and EMC).

Hybrid substrate 500 is electrically interconnected to electricalterminals 410 through wire bonds 530, though it is well understood thatany of a large number of electrical interconnection techniques would besuitable. Electrical connector terminals 410 emerge from housing 300 ata connector body 400, for interconnection to standard mating connectors.

Magnetic structure 200 rotates about the generally center axis 250relative to housing 300, thereby rotating magnets 212 and 214 togetherwith pole piece 210. Hall effect device 510 is stationary relative tothe housing 300. Best illustrated in FIG. 3, magnets 212 and 214 areshaped generally helically so as to have a relatively thicker end and arelatively thinner end. At the thicker ends 211 and 215, which is at thesame angle of rotation of magnetic structure 200 for both magnets 212and 214, there is a narrow air gap 217. At the thinner ends 213 and 216,there is a correspondingly wide air gap 218. The result is thegeneration of less magnetic induction across gap 218, with more magneticinduction across gap 217.

Rotation of pole piece 210 about axis 250 results in changing fieldmagnetic induction which is directly measured by Hall effect device 510.Proper shaping of the gap will produce a linear output from Hall effectdevice 510. However, such a system will not perform linearly and withprecision and resistance to bearing play over its life without furtherdesign considerations.

In order to stabilize a magnet against irreversible losses, it isnecessary first to saturate magnets 212 and 214 and then to demagnetizethe magnets by a small amount. The magnetic structure 200 does notdemagnetize evenly from magnet ends 211 and 215 to magnet ends 213 and216, without special consideration. Absent the appropriatedemagnetization, described in our copending application Ser. No.08/223,474 filed Apr. 5, 1994 and copending herewith and incorporatedherein by reference, the resulting device will either lose precision asa result of temperature excursions or will lose linearity as a result ofstabilizing demagnetization.

FIGS. 4, 5 and 6 illustrate an alternative embodiment to magnetstructure 200, with rotor cup 220 removed for clarity. Therein, magnetstructure 450 includes a magnetically permeable pole piece 460 and twoshaped magnets 464 and 466. Magnets 464 and 466 do not have the lips ofthe preferred embodiment. In every other way, this structure is designedto be a functional equivalent, of the FIGS. 1-3 embodiment with aslightly reduced "sweet zone" of operation. The magnets 464 and 466 arestill tapered so as to provide a changing magnetic induction withrotation.

FIGS. 7 and 8 illustrate an alternative embodiment of cover 310, whereina ferromagnetic plate 814 is shown molded into cover 810. Cover 810includes reinforcing ribs 812 similar to ribs 312. The use of aferromagnetic plate further reduces the sensitivity of position sensor100 to external ferromagnetic objects, for those applications requiringextreme precision. For EMC and EMI considerations, plate 814 should begrounded.

The apparatus for measuring angular or rotary position described hereinas preferred is a low cost structure due to the minimal weight andreduced demands upon magnetic components. In addition, there are manyperformance advantages not heretofore obtainable, including reducedsensitivity to bearing play, resistance to contamination andenvironment, reduced sensitivity to externally located fields, energiesand objects, durability for both full stroke motion and dithers,precision, linearity, reduced complexity, and reduced cost.

While the foregoing details what is felt to be the preferred embodimentof the invention, no material limitations to the scope of the claimedinvention is intended. Further, features and design alternatives thatwould be obvious to one of ordinary skill in the art are considered tobe incorporated herein. The scope of the invention is set forth andparticularly described in the claims hereinbelow.

We claim:
 1. In combination, a rotary sensor for use with an internalcombustion engine comprising;a) a throttle operatively connected to theengine; b) the rotary sensor including an assembly for providing aclosed magnetic circuit including:b1) a first magnet; b2) a secondmagnet; b3) a magnetically permeable pole piece interconnecting thefirst and second magnets; and b4) a varying dimension air gap definedbetween the first magnet and the second magnet; b5) the first and secondmagnets being structured and dimensioned to provide the varyingdimension air gap and to form a variable magnetic field coupledtherebetween; b6) the assembly being coupled to the throttle and beingrotatable about an axis generally parallel to the variable magneticfield coupled between the magnets so that rotation of the throttlecauses rotation of the assembly; and b7) the first magnet having a firstinner radial edge positioned essentially coextensive with the axis and afirst outer edge radially spaced from the axis, the second magnet havinga second inner radial edge positioned essentially coextensive with theaxis and a second outer edge radially spaced from the axis, the firstand second inner edges and the first and second outer edges of the firstand second magnets, respectively, being congruent; and c) a magneticfield sensing means positioned in the air gap for sensing the variablemagnetic field coupled between the first and second magnets, themagnetic field sensing means being positioned at a location between thefirst and second inner edges and the first and second outer edges of thefirst and second magnets.
 2. The rotary position sensor of claim 1wherein the first outer edge and the second outer edge are essentiallyequidistant from the axis.
 3. The rotary position sensor of claim 1wherein the magnetic circuit assembly is mounted so that the magnetsrotate in planes generally perpendicular to the axis.
 4. The rotaryposition sensor of claim 1 wherein the field sensing means is a Halleffect device.
 5. The rotary position sensor of claim 1 wherein thefirst and second magnets are each shaped generally helically so as tohave a relatively thicker end and a relatively thinner end.
 6. Therotary position sensor of claim 5 wherein said thicker ends of saidfirst and second magnets are arranged so as to provide a narrow air gapbetween the magnets relative to a wide air gap between the thinner endsof the magnets.
 7. The rotary position sensor of claim 1 wherein theassembly or providing a closed magnetic circuit is generally C-shaped incross-section with the first and second magnets extending outwardly fromthe interconnecting magnetically permeable pole piece.
 8. The rotaryposition sensor of claim 7 wherein the pole piece is formed from amaterial selected from the group consisting of molded and sinteredmetals and sheet steel.
 9. The rotary position sensor of claim 7 whereinthe first and second magnets are formed from a ferromagnetic material.10. In combination, a rotary position sensor for use with an enginehaving a rotatable part to be monitored and including a shaft connectedto the part and to the sensor comprising:a) a rotatable part connectedto the engine; b) a rotatable closed magnetic circuit assembly connectedto the engine and connected to the shaft so that rotation of the partcauses rotation of the magnetic circuit assembly; c) the closed magneticcircuit assembly includes a first magnet, a second magnet, a C-shapedmagnetically permeable pole piece and a varying dimension air gapforming a variable magnetic field coupled between the first and secondmagnets; d) the pole piece has two arms and a base and the first andsecond magnets are connected to the arms of said pole piece forming theair gap therebetween; e) the interconnected first and second magnets aremounted for rotation with the pole piece about an axis; f) the firstmagnet having a first inner radial edge positioned essentiallycoextensive with the axis and a first outer edge radially spaced fromthe axis, the second magnet having a second inner radial edge positionedessentially coextensive with the axis and a second outer edge radiallyspaced from the axis, the first and second inner edges and the first andsecond outer edges of the first and second magnets, respectively, beingcongruent; and g) a stationary magnetic field sensing Hall effect devicepositioned in the air gap between the first magnet and the second magnetfor sensing the variable magnetic field generally parallel to the axis,the magnetic field sensing Hall effect device being positioned at alocation between the first and second inner edges and the first andsecond outer edges of the first and second magnets.
 11. The rotarysensor of claim 10 wherein the interconnected first and second magnetsare mounted for rotation in planes generally perpendicular to the axis.12. The rotary sensor of claim 11 wherein the first and second magnetshave equidistant outer edges spaced radially outwardly from the axis.13. The rotary sensor of claim 12 wherein said first and second magnetsare each shaped generally helically so as to have relatively thickerends and relatively thinner ends, and the thicker ends of the first andsecond magnets are arranged so as to provide a narrow air gap betweenthe magnets relative to a wide air gap between the thinner ends of themagnets.